The present invention relates to a reflectron for a time-of-flight mass spectrometer, and more specifically an atom probe microscope.
Time-of-flight mass spectrometers typically include a specimen, a means to generate and liberate ions from the specimen and an electric field to attract these liberated ions to a detector. A means to measure the time between the initial ion liberation and the detection of the ion enables the measurement of transit time. The transit time is proportional to the mass-to-charge ratio of the ion, hence information about the atomic composition of the specimen can be determined.
These liberated ions have neither the same starting time nor the same kinetic energy. The spread in starting times is a function of the width of the initial ionizing pulse mechanism. The spread in kinetic energies for these ions results from the time-varying evaporation field present during ionization as well as the initial specimen geometry.
Time-of-flight mass spectrometers may incorporate a reflectron to improve the mass resolution of the device. The reflectron effectively acts as an electrostatic ‘mirror’, and alters the flight path of an ion which is being analyzed in the mass spectrometer. The ion is deflected from its initial direction from an ion source onto a detector.
A conventional reflectron is formed of a series of primarily planar ring electrodes, which define a hollow cylinder. The electrodes are each held at an electric potential, the potential increasing in a direction of travel of an ion from an ion source. The electrodes generate a uniform field over the cross-section of the reflectron. Indeed, the flatness of the fields is a key design criterion for conventional reflectrons. Any residual curvature of the fields, which is difficult to avoid, leads to aberrations in ion trajectories and degradation in mass resolution. The ions travel in a parabolic path through the reflectron. Ions with more kinetic energy travel farther into the reflectron, hence their path length is longer and their transit time to the detector is longer. Ions with less kinetic energy do not travel as deep, traverse a shorter path, and have shorter transit times. It can be deduced that ions with a given mass-to-charge ratio and varying kinetic energies will have less variation in their transit time, hence the measured mass resolution will be improved. The reflectron can be configured such that the time taken by the ion to travel through the atom probe is substantially independent of the initial energy of the ion. This is known as time focusing.
Ions liberated with the same mass-to-charge ratio but slightly different kinetic energies will follow different trajectories through the reflectron and will strike the detector at slightly different locations. The spread of impact positions is proportional to the chromatic aberration of the system. In addition, as the field of view (FOV) increases so does the chromatic aberration.
A reflectron with a curved rear electrode is evident in U.S. Pat. No. 6,740,872. In this embodiment, the curved electrode serves to space-angle focus a slightly-divergent point source to a point collector which improves the coupling efficiency between source and detector. There is no intent or prospect to collect information about the angular variation in intensity across the source, i.e., to resolve an image. Other embodiments (EP 0 208 894, U.S. Pat. No. 4,731,532) accomplish similar effects but with lesser operational flexibility. Keller and Srama et al. describe reflectrons that include dual shaped grids, but images are not being resolved.
The reflectron can increase the mass resolution of an atom probe microscope in a similar way to its use in a time-of-flight mass spectrometer. Further advances enable use of a reflectron in a three-dimensional atom probe—a microscope that yields atomistic imaging with spectroscopic information. What follows is a description of that particular embodiment.
The ion source in an atom probe microscope is a specimen under examination with a curved surface of small dimensions. The ions originate from a small area of the surface and proceed towards a detector at some distance away. They can form an image of the sampled area at a very large magnification if a position-sensitive detector is utilized. High mass resolutions are possible with small FOV configurations, while lower mass resolutions are possible with wider FOV arrangements.
While a conventional reflectron incorporated in an atom probe can increase the measured mass resolution it has the disadvantage that an angle spread of more than approximately 8 degrees results in an excessively large reflectron and detector or alternatively an excessively short flight path, hence the FOV is limited.
Another disadvantage of a conventional reflectron is that chromatic aberration results in a positioning error at the detector that increases with angle away from the reflectron's normal. Chromatic aberration is an error in the imaged position of the detected ion and is a function of the energy of the ion. The FOV of an atom probe employing a conventional reflectron to increase mass resolution is therefore usually limited to relatively small angles (approximately 8° included angle).
A reflectron used in a three-dimensional atom probe must accept ions over a significantly larger range of angles than a reflectron in a time-of-flight mass spectrometer. A reflectron designed for use in a traditional atom probe or a time-of-flight mass spectrometer will not be suitable for use in a three-dimensional atom probe if they will only accept and reflect ions incident over a small range of angles.